Development of Organocatalysts for Ring-Opening Polymerization (ROP) of Lactones
Organocatalysis for Ring-Opening Polymerization (ROP) has come a long way in recent developments to afford precisely tailored and highly adorned biodegradable polyesters. A remarkable milestone of the organocatalysts occurred in 2005 with the advent of dual H-bonding catalysts that produce superior reaction control and molecular weight distributions (Mw/Mn), which is ideal for material applications. However, these organocatalysts do not show the capability in faster reaction times which is limiting the feasibility for industrial implementation. The polymerization of cyclic esters by (thio)urea/base cocatalyst has proved to be effective and controlled. One method of devising improved catalyst systems is through mechanistic investigations. It has shown that ROP can proceed via one of two mechanisms: Neutral H-bonding mechanism and (thio)imidate mediated mechanism. It has been found that (thio)imidate mechanism is preferred reaction conditions such as polar solvents, high temperature, high monomer concentration, presence of strong electron-withdrawing groups on the H-bond donor, and strong bases which resulted in effective ROP kinetics and precisely tailored polymers. The synthetic addition of one or more (thio)urea H-bond donating arms to the parent (thio)urea has been shown to substantially increase the activity of (thio)urea H-bond donors. A series of conformationally flexible bis(thio)urea H-bond donors plus base cocatalyst were applied to understand the structure-function relationship of the multi H-bonding (thio)ureas in the ring-opening polymerization of lactones. The rate of the ROP displays a strong dependence upon the length and identity of the tether, where a circa five methylene-unit long tether exhibits the fastest ROP of δ-valerolactone (VL) and ε-caprolactone (CL), which could be accelerating reaction rates from days to seconds, and remains active at low catalyst loadings under solvent-free conditions. An extensive kinetic study was carried out with ROP of VL employing multi H-bonding urea catalysts in polar solvents. It has revealed that multiple urea moieties in the catalysts facilitate activation of several monomers, which resulted in higher-order kinetics in monomer; hence, higher initial rates in ROP reactions were observed. It is also found that the polymer architecture could be modified in copolymerization due to higher-order kinetics in VL with multi H-bonding urea catalysts in polar solvents. For the first time, organocatalytic ring-opening polymerization (ROP) of thiono-macrolactones was conducted. The ROP of less strained (thiono)macrolactones showed entropic driving force for the reaction with minimal or negligible contribution from enthalpy for the ROP yet, retain the characteristics of living polymerization even at elevated temperatures. The copolymerization of thionomacrolactones and macrolactones showed altered material properties compared to its homopoly(thiono)lactones.
Organic chemistry|Polymer chemistry|Physical chemistry|Materials science|Kinesiology
"Development of Organocatalysts for Ring-Opening Polymerization (ROP) of Lactones"
Dissertations and Master's Theses (Campus Access).